In recent, the demands on portable electronic products such as notebooks, video cameras and cellular phones are rapidly increased, and the development of electric vehicles, energy storage batteries, robots and satellites is under active progress. Accordingly, high-performance secondary batteries capable of being repeatedly charged are vigorously researched.
Currently, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries and lithium secondary batteries are commercially available as secondary batteries. Among them, lithium secondary batteries are spotlighted since they may be freely charged or discharged since memory effects are scarcely caused in comparison to nickel-based secondary batteries. In addition, lithium secondary batteries exhibit very low self-discharge and high energy density.
Meanwhile, the most important issue in relation to batteries is the safety. In particular, notebooks and cellular phones are used more and more, and if a battery is exploded, a portable electronic product having the battery may be broken and even cause a fire. Thus, the safety of a battery should be ensured. Until now, various kinds of protecting apparatuses have been used to ensure the safety of a battery. Such a protecting apparatus intercepts a charging/discharging current to ensure the safety of a battery if an abnormal state was detected in a battery. In generally, the battery pack protecting apparatus includes a fuse for irreversibly breaking a line along which a charging/discharging current is flowing so as to intercept the current flow and thus protect a battery pack in case an abnormal state such as overcharging, overdischarging, short circuit and overcurrent happens, and a sense resistor for detecting a magnitude of the charging/discharging current.
FIG. 1 is a block diagram schematically showing a conventional battery pack protecting apparatus.
Referring to FIG. 1, a conventional battery pack protecting apparatus 100 is connected to a cell group 20 composed of at least one battery cell, and the conventional battery pack protecting apparatus 100 includes a high voltage output terminal Pack+ and a low voltage output terminal Pack− connected to a charging device or a load, a charge control switch 60 and a discharge control switch 50 respectively connected to a high voltage line in series to intercept a charging current and a discharging current when overcharging or overdischarging occurs, a sense resistor 10 connected to a low voltage line in series to sense a current flowing in the protecting apparatus 100, a fuse 40 for irreversibly breaking a line along which a charging/discharging current is flowing in case an abnormal state such as overcharging and overdischarging occurs in a battery, a fuse control switch 30 for selectively opening or closing the flow of current used for operating the fuse 40, a first protective circuit 80 for sensing an individual cell voltage and outputting the sensed voltage to a microcontroller 90, a second protective circuit 70 for sensing an individual cell voltage and applying an operation voltage to the fuse control switch 30 to melt and cut the fuse 40 when an overcharging state over a critical value is detected, and a microcontroller 90 for receiving the voltage of each cell from the first protective circuit to monitor whether the entire cells or individual cells are overcharged, sensing a charging/discharging current of the battery through the sense resistor 10 to monitor whether overcurrent occurs, and, when an abnormal state such as overdischarging or overcurrent is detected, controlling ON/OFF operation of each switch 30, 50, 60 to temporarily or permanently intercept the flow of charging/discharging current.
The charge control switch 60, the discharge control switch 50 and the fuse control switch 30 are made of FET (Field Effect Transistor) having a parasitic diode therein. The charge control switch 60 and the discharge control switch 50 receive an operation voltage at their gate terminals from the first protective circuit 80 under the control of the microcontroller 90. Also, the fuse control switch 30 receives an operation voltage at its gate terminal from the microcontroller 90 or the secondary protective circuit 70.
In addition, the fuse 40 is configured as a three-terminal fuse. In case an individual cell or a cell group 20 is overcharged or overdischarged beyond a limit since the charge control switch 60 or the discharge control switch 50 is damaged not to normally control charging or discharging, in case an overcurrent is flowed since a short circuit is caused in a load or a protecting apparatus, or in case an abnormal state such as abrupt increase or decrease of a charging voltage happens since an individual cell is damaged, the fuse 40 irreversibly breaks the line along which the charging/discharging current flows (namely, the line of the Pack+ terminal) to prevent explosion of the battery pack, permanent damage of the battery cell, or breakdown of a load connected to the battery pack.
Meanwhile, as the power consumption of a battery pack demanded in a high-power battery system is increased recently, the capacities consumed by safety-related parts used in the battery pack are also increased, and accordingly a current capacity of the three-terminal fuse also tends to increase from 7 A to 12 A. If an external short circuit or an overcurrent occurs in this circumstance, the fuse may be melted and cut later, so important parts located on a main current path may be broken down before the fuse is melted and cut. In particular, if the sense resistor 10 serving an important role in the battery pack protecting apparatus is broken due to overcurrent or the like, a great voltage is applied to both ends of the sense resistor 10, so an overvoltage exceeding an allowable limit may be applied to the microcontroller 90. In this case, the microcontroller 90 may be damaged not to apply an operation voltage to the fuse control switch 30 and thus not to melt and cut the fuse 40, which is essential for the protecting operation. As a result, a little current is consistently flowed until the battery is entirely discharged, and in this process, a battery temperature is increased, which may cause even a fire to the battery in a serious case.